AlterModelManagers migration is generated for all Models with custom Managers, which is not compatible with Django 1.8

[Michał Bońkowski]

We have a Model and a default_manager with a custom name:

class MyManager(models.Manager):
    use_in_migrations = False

class MyModel(models.Model):
    my_field = models.BooleanField()

    class Meta:
        default_manager_name = 'my_objects'

    my_objects = MyManager()

This creates a migration with a default manager though no manager is marked with use_in_migrations = True.

class Migration(migrations.Migration):

    operations = [
        migrations.CreateModel(
            name='MyModel',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('my_field', models.BooleanField()),
            ],
            options={
                'default_manager_name': 'my_objects',
            },
            managers=[
                ('my_objects', django.db.models.manager.Manager()),
            ],
        ),
    ]

According to https://code.djangoproject.com/ticket/26643#comment:5, we should not get this manager added to the state.

After some investigation, we can see that manager with any other name than objects will be added to the migration: ​https://github.com/django/django/blob/1.11.8/django/db/migrations/state.py#L537.